This invention relates generally to systems for filtering signals, and more particularly, to systems for filtering signals, especially signals received in medical imaging systems.
Ultrasound imaging is used in a variety of clinical settings, including, for example, obstetrics, gynecology, cardiology and oncology. Ultrasound imaging is widely used to study anatomical structures, detect anomalies in tissues and measure blood flow within the body. In ultrasound imaging systems, a transducer probe of an ultrasound machine generates and transmits acoustic waves and receives the echoes reflected, for example, by a body or portion thereof.
Doppler ultrasound, which is based upon the Doppler effect, is used for measuring the rate of blood flow through the human body, for example, through the heart, major arteries and veins in the body. In accordance with the Doppler effect, the frequency of reflected echoes from a moving object are different from the frequency of the transmitted waves. The frequency of the echoes is higher than that of the transmitted waves if the object is moving towards the probe and vice versa.
Doppler ultrasound measures the change in frequency of the echoes to calculate the flow velocity of a body fluid such as blood. The velocity of blood is not same throughout the flow region, for example, through the blood vessels carrying the blood. Typically, the velocity follows a parabolic profile, being highest at the center of the blood vessels and decreasing towards the walls of the vessels. Signals originating from the stationary and slow moving tissues, such as vessel walls, have a lower Doppler frequency shift. The vessel wall signal is typically 40 to 100 db stronger than the signal from the blood. Without sufficient wall signal rejection, low velocity blood flow cannot be measured or detected.
Ultrasound systems use a high pass filter, sometimes referred to as a vessel wall filter, to remove the low frequency tissue motion signal in blood flow velocity estimations. When both B-mode image and flow image, including spectral Doppler imaging and color flow imaging, are active, ultrasound systems transmit acoustic waves alternatively for flow imaging and B mode imaging. The vessel wall filter is turned on in each flow segment. The abrupt turn-on at the beginning of each Doppler segment may introduce transient noise. The transient noise obscures the low amplitude Doppler signal from the blood flow, which can cause difficulty for some diagnoses based on velocity measurements. This transient noise also may corrupt the mean velocity estimation in color flow imaging. It is very important for the wall filter to efficiently remove the low frequency tissue motion signal without introducing transient noise in Doppler frequency estimation.
Currently, Infinite Impulse Response (IIR) filters and Finite Impulse Response (FIR) filters are used to remove these low frequency signals. IIR filters can provide sharper roll-offs at cutoff frequencies with fewer sample points. However, these IIR filters may introduce large transient artifacts. The artificial signals shift to higher frequency with higher cutoff. Further, FIR filters require a long filter length to achieve sharp roll-offs. However, the sample points limit the actual filter length in the case of FIR filters. Because of the slow roll-off, the attenuation may not be adequate to remove the much stronger low velocity tissue motion signal for a lower cutoff filter. In addition, the higher cutoff filter may remove too much of the blood flow signals. FIR filters also may introduce transient noise that may be observed as multiple narrow frequency bands spreading from the baseline into higher frequencies. These transient noises are stronger with stronger tissue wall signals. Thus, the transient signals and the tissue wall signals obscure or interfere with the actual low frequency flow signals and may cause the flow signals to be undetectable. This also may result in inaccurate mean velocity estimation.
Thus, known ultrasound filters may not provide efficient removal of low frequency tissue motion signals and result in some transient noises. These filters may also obscure or interfere with the detection of low velocity flow both in Doppler spectral waveform and color flow imaging.